It's believed to be the first successful case of a transgender woman breastfeeding an infant.

"It's out there on internet forums, but there's a lot on the internet that's true or untrue to varying degrees," Assoc Prof Safer told New Scientist.

Whether it worked because of the hormones, the breast pump or a combination of both remains to be proven. The US Food and Drug Administration warns against using domperidone as a milk production stimulant because of its unknown effects on infants, and "its association with cardiac arrhythmias, cardiac arrest, and sudden death".

The contents of the milk are yet to be assessed, but the baby's good health was confirmed by a paediatrician. The long-term effects, if there are any, remain unknown.

Dr Reisman and Dr Zil hope other transgender women will be able to breastfeed their babies in the future without fear of ill-effects on themselves or their children.

"Breastfeeding offers immunological, metabolic, and psychosocial benefits for both mother and infant… economic advantages by allowing families to save resources that might be devoted to formula and infant healthcare... [and] has been noted to facilitate mother-child bonding."